Add info about I18n::Backend::Chain to I18n guide

railties/guides/source/i18n.textile

@@ -809,6 +809,12 @@ That does not mean you're stuck with these limitations, though. The Ruby I18n ge
 I18n.backend = Globalize::Backend::Static.new
 </ruby>
 
+You can also use the Chain backend to chain multiple backends together. This is useful when you want to use standard translations with a Simple backend but store custom application translations in a database or other backends. For example, you could use the ActiveRecord backend and fall back to the (default) Simple backend:
+
+<ruby>
+I18n.backend = I18n::Backend::Chain.new(I18n::Backend::ActiveRecord.new, I18n.backend)
+</ruby>
+
 h4. Using Different Exception Handlers
 
 The I18n API defines the following exceptions that will be raised by backends when the corresponding unexpected conditions occur: